Much attention has been given in recent years to the development of ultrasonic systems for producing real time images of internal portions of the human body. In one system, an array of transducers for converting short pulses of electrical alternating current carrier waves into corresponding pressure waves is placed in contact with the body. By choosing the relative times of application of the pulses of electrical carrier waves to the transducers, the pressure waves for each pulse can be formed into a beam extending in any desired direction, and the beam direction can be changed so as to effectively scan a sector. As the pulses of pressure waves pass through the body, a portion of their energy is reflected whenever they encounter tissue having a different acoustic characteristic. An array of receiving transducers is provided for converting the reflected pressure waves into corresponding electrical waves.
Precise focussing of the array of receiving elements at a given point requires that all of the few cycles of the alternating current waves derived by each of the transducers from a pulse of pressure waves reflected from that point be superimposed in time synchrony at a summing point so that the pulses are perfectly aligned. This produces a strong signal whereas reflections of pressure waves from other points produce weak signals because the corresponding electrical waves arrive at the summing point with random phase relationships. The distances between any desired focal point and the various receiving transducers being different, the reflections arrive at the transducers at different times. It is therefore necessary in order to achieve precise focussing to introduce compensating delays between each transducer and the summing point so that the total time between reflections of a pressure wave at the focal point and the arrival of the corresponding electrical wave at the summing point is the same regardless of which transducer is involved. The compensating delays may be varied so that the focal point is dynamically scanned from minimum to maximum range along each direction of the transmitted pulses.
In some present equipment, the variations in compensating delays are achieved by changing taps on delay lines. The taps cannot be more than a small fraction of the period of a carrier wave apart if the cycles of the carrier wave are to arrive at the summing point nearly in phase. Inasmuch as the total change in compensating delay for some transducers as they are focussed from minimum to maximum range and from minimum to maximum sector angle is equal to many periods of the carrier wave, the number of taps required is large. At the carrier frequencies employed, only the relatively expensive electrical delay line can be used because of bandwidth considerations, and the provision of a large number of taps on this type of line is a significant portion of the cost of the entire instrument.
Of equal significance is the fact that, unless expensive tap changing switches are used, the switching transients cause a significant amount of noise in the signals arriving at the summing point and therefore in the image produced from them.
The problems of cost and transient noise just referred to increase in severity when the carrier frequency is increased to obtain better definition, the aperture of the array is increased to obtain better focussing, or the minimum range is decreased so as to permit the examination of infants.